Rotary paddle sensors are currently used to monitor the level of dry bulk materials in storage bins, grain hoppers, silos, and other structures. A bin level indicator usually consists of a small electric motor mounted in a housing, attached to a paddle with one or more vanes that drives the paddle. The paddle is able to freely rotate in absence of material. But when the paddle is impeded by material, the motor rotates within a housing which triggers a mechanical switch. The switch turns off the motor and also gives an indication that the material has reached the level of the paddle. U.S. Pat. Nos. 2,851,553, 3,941,956, and 4,095,064 describe such mechanism which responds to stalling of the motor due to the level of the bulk material.
U.S. Pat. No. 4,211,966 describes an improvement using a spring coupling which provides a time delay feature, but the invention still utilizes the stalling of the motor. U.S. Pat. No. 4,318,624 describes a slip clutch mechanism between the paddle and the motor, but still relies on movement of the motor to engage a mechanical switch. The slip clutch prevents possible damage to the electric motor due to the stoppage of the paddle if the switches do not turn off the motor.
U.S. Pat. Nos. 4,827,768 and 4,884,444 show multiple paddles on a driven shaft to indicate a level of a fluid, but the paddles are fixed to the driven shaft and the level is determined by the torque needed to overcome the fluid resistance. The invention would not work with dry bulk material such as grain or with other solid matter.
U.S. Pat. No. 4,375,020 describes a friction clutch that isolates the motor from the paddle, but the invention also relies on the rotation of the motor to engage a mechanical switch. U.S. Pat. No. 6,696,965 employs the same technique as in the previously described designs, but instead of using mechanical switches U.S. Pat. No. 6,696,965 uses a magnetic pickup to indicate the movement of the motor.
The past rotary paddle sensors have several drawbacks. They rely on the movement of the electric motor to engage mechanical contact switches which are prone to wear out and fail, causing the electric motor to overheat. The movement of the electric motor within housing requires intricate mechanical design and is also prone to wearing out. The bin level rotary paddle designs are limited to indicate one specific level since the design relies on a close coupling between a single motor and the paddle. In the multiple paddle designs, the designs are limited to determining only fluid level.
There is consequently a need for a form of rotary paddle sensor that eliminates reliance on movement of the electric motor itself which would simplify the mechanical design and allow for use of other driving mechanisms. There is also a need to eliminate any possible overheating of the motor and failure of the mechanical switches. Moreover, it would be desirable for such a sensor to have the ability to indicate more than one specific level.